Obesity represents the most prevalent of body weight disorders, affecting an estimated 30 to 50% of the middle-aged population in the western world. Obesity, defined as a body mass index (BMI) of 30 kg/2m or more, contributes to diseases such as coronary artery disease, hypertension, stroke, diabetes, hyperlipidemia and some cancers. (See, e.g., Nishina, P. M. et al. (1994), Metab. 43:554-558; Grundy, S. M. & Barnett, J. P. (1990), Dis. Mon. 36:641-731). Obesity is a complex multifactorial chronic disease that develops from an interaction of genotype and the environment and involves social, behavioral, cultural, physiological, metabolic and genetic factors.
Generally, obesity results when energy intake exceeds energy expenditure, resulting in the growth and/or formation of adipose tissue via hypertrophic and hyperplastic growth. Hypertrophic growth is an increase in size of adipocytes stimulated by lipid accumulation. Hyperplastic growth is defined as an increase in the number of adipocytes in adipose tissue. It is thought to occur primarily by mitosis of pre-existing adipocytes caused when adipocytes fill with lipid and reach a critical size. An increase in the number of adipocytes has far-reaching consequences for the treatment and prevention of obesity.
Adipose tissue consists primarily of adipocytes. Vertebrates possess two distinct types of adipose tissue: white adipose tissue (WAT) and brown adipose tissue (BAT). WAT stores and releases fat according to the nutritional needs of the animal. This stored fat is used by the body for (1) heat insulation (e.g., subcutaneous fat), (2) mechanical cushion (e.g., surrounding internal organs), and (3) as a source of energy. BAT burns fat, releasing the energy as heat through thermogenesis. BAT thermogenesis is used both (1) to maintain homeothermy by increasing thermogenesis in response to lower temperatures and (2) to maintain energy balance by increasing energy expenditure in response to increases in caloric intake (Sears, I. B. et al. (1996) Mol. Cell. Biol. 16(7):3410-3419). BAT is also the major site of thermogenesis in rodents and plays an important role in thermogenesis in human infants. In humans, and to a lesser extent rodents, brown fat diminishes with age, but can be re-activated under certain conditions, such as prolonged exposure to cold, maintenance on a high fat diet and in the presence of noradrenaline producing tumors.
Fat metabolism is regulated by two pathways, lipogenesis and lipolysis. Lipogenesis is the deposition of fat which occurs in the liver and in adipose tissue at cytoplasmic and mitochondrial sites. This process allows the storage of energy that is ingested which is not needed for current energy demands. Lipolysis is the chemical decomposition and release of fat from adipose and/or other tissues. This process predominates over lipogenesis when additional energy is required by the body.
Any treatment for obesity has to reduce energy intake, increase energy expenditure or combine both effects. Current therapies for obesity predominantly lead to decreased energy intake by acting at satiety centers in the brain or by reducing the efficiency of intestinal absorption. To date, no safe and reliable molecular mechanism for treating and/or preventing obesity by increasing energy expenditure or metabolic activity has been identified. Given the severity and prevalence of obesity related disorders, there exists a great need for the identification of an anti-obesity therapeutic.